Remove unused method 'string_to_bool' from utils

The method 'string_to_bool' in utils is not used.
Furthermore, if we need this method in the future,
we should use 'bool_from_string' from oslo cliutils
to replace it.

partially implements blueprint common-client-library-2

Change-Id: I21978d389a8721a40e1669351575297ce61a438e

heatclient/common/utils.py

@@ -116,10 +116,6 @@ def find_resource(manager, name_or_id):
         raise exc.CommandError(msg)
 
 
-def string_to_bool(arg):
-    return arg.strip().lower() in ('t', 'true', 'yes', '1')
-
-
 def env(*vars, **kwargs):
     """Search for the first defined of possibly many env vars